text:
Which regulations require the ARROW documents?
In the US, a common mnemonic for the documents that must be on board an aircraft is ARROW:

Airworthiness Certificate
Registration Certificate
Radio Station License (international flights only)
Operating Handbook
Weight and Balance

Which regulations state that these documents must be on board?

Not a regulation, but CHAPTER 56   CONDUCT A FAR PART 91 RAMP INSPECTION lists the items that an FAA inspector looks for when doing a ramp check. http://fsims.faa.gov/WDocs/8700.1%20GA%20Ops%20Insp%20Handbk/Volume%202/2_056_00.htm

It is in several places:

Airworthiness Certificate  FAR 91.203
Registration Certificate FAR 91.203
Radio Station License (international flights only)
Operating Handbook FAR 91.9
Weight and Balance FAR 23.1589, now moved and reworded to FAR 23.2620 since c. mid-2017

CFR 91.9:

(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.
(b) No person may operate a U.S.-registered civil aircraft -
  (1) For which an Airplane or Rotorcraft Flight Manual is required by § 21.5 of this chapter unless there is available in the aircraft a current, approved Airplane or Rotorcraft Flight Manual or the manual provided for in § 121.141(b); and
  (2) For which an Airplane or Rotorcraft Flight Manual is not required by § 21.5 of this chapter, unless there is available in the aircraft a current approved Airplane or Rotorcraft Flight Manual, approved manual material, markings, and placards, or any combination thereof.

CFR 23.1589:

The following loading information must be furnished:
(a) The weight and location of each item of equipment that can be easily removed, relocated, or replaced and that is installed when the airplane was weighed under the requirement of § 23.25.
(b) Appropriate loading instructions for each possible loading condition between the maximum and minimum weights established under § 23.25, to facilitate the center of gravity remaining within the limits established under § 23.23.

FAR 91.203

(a) Except as provided in § 91.715, no person may operate a civil aircraft unless it has within it the following:
(1) An appropriate and current airworthiness certificate. Each U.S. airworthiness certificate used to comply with this subparagraph (except a special flight permit, a copy of the applicable operations specifications issued under § 21.197(c) of this chapter, appropriate sections of the air carrier manual required by parts 121 and 135 of this chapter containing that portion of the operations specifications issued under § 21.197(c), or an authorization under § 91.611) must have on it the registration number assigned to the aircraft under part 47 of this chapter. However, the airworthiness certificate need not have on it an assigned special identification number before 10 days after that number is first affixed to the aircraft. A revised airworthiness certificate having on it an assigned special identification number, that has been affixed to an aircraft, may only be obtained upon application to an FAA Flight Standards district office.
(2) An effective U.S. registration certificate issued to its owner or, for operation within the United States, the second copy of the Aircraft registration Application as provided for in § 47.31(c), a Certificate of Aircraft registration as provided in part 48, or a registration certification issued under the laws of a foreign country.

The other "R" for the radio station license was in the FCC documents, but since it is no longer required, it does not appear. If you are operating within the United States, the acronym is AROW. Outside of the United States you would have to see the Canadian, Mexican, or other countries requirements. Here is the Canadian information as an example.

Part 23 says that a W&B must be furnished by the manufacturer—not that it be in the aircraft. There are several FARs, especially §91.103   Preflight action.,  that require you to consult the W&B information before flight. But there is nothing that requires a Part 91 pilot to have a current W&B in the aircraft. As a practical matter, everyone I know keeps it in the aircraft.

@JScarry Part 23.1581-1589 identifies what must be included in the AFM, read it together with 91.9 and it becomes a required document carried on board.

Neither of my airplanes is required to have a §21.5  "approved Airplane or Rotorcraft Flight Manual"  ergo, they are not required to have a W&B in the plane. You could also argue that "§23.1589 (a) The weight and location of each item of equipment that can be easily removed, relocated, or replaced and that is installed when the airplane was weighed under the requirement of §23.25." does not require that an updated W&B be included in the AFM only that one must be provided by the manufacturer.

@JScarry I guess you can argue that, but I'm not an aviation attorney. All I know is that every time we have an annual the W&B is verified and updated if necessary in the AFM supplement in the aircraft, and when we did an avionics upgrade in February the avionics shop did a new W&B and specifically instructed us to keep it in the AFM binder in the aircraft. No, not all aircraft are required to have an AFM, and those exceptions are noted in the CFR's.

Good answer, and if you can update it to make the 'W' a bit clearer then I'll accept: the point about 23.1589 being related to 91.9 isn't obvious from the answer, as @JScarry commented. Your response addresses that but it would be helpful to have it in the answer itself.

The ARROW acronym—Airworthiness Certificate, Registration, Radio License, Operating Limitations, Weight and Balance is fairly straight forward until you get to the Operating Limitations and Weight and Balance part. Then there it gets a little complicated.
There are three occasions when you may be asked for these documents. When being ramp checked, when the aircraft goes in for an annual inspection, or when the aircraft is involved in an accident or incident.
CHAPTER 56 CONDUCT A FAR PART 91 RAMP INSPECTION

AIRCRAFT DOCUMENTS. Following are considerations when examining
aircraft documents, including registration and airworthiness
certificates and approved flight manuals. Discrepancies found
concerning the airworthiness or registration certificates shall be
brought to the attention of the operator, documented, and given to the
airworthiness unit for action.
A. N-Numbers. The N-number on the registration certificate must match
the N-number on the airworthiness certificate.
B. Registration Certificate. If the registered owner has
changed you may see a temporary registration (Pink Slip) which is good
for 120 days. If the ownership has changed without a Pink Slip or the
N- numbers do not match, the registration is not valid.
C. Radio Station License.
An aircraft FCC radio license is required although
the FAA does not regulate the requirement. The license may be for that
particular N-number or a fleet license. The expiration date of the
license is in the upper right hand corner. Any discrepancy concerning
the radio license should be brought to the attention of the operator
only.
D. Flight Manual. An Aircraft Flight Manual is required to be on
board the aircraft (FAR § 91.9 {91.31}) along with the appropriate
markings and placards.
E. Weight and Balance Information. Weight and
balance documents, including a list of equipment, must be on board the
aircraft. Some multiengine operators have Minimum Equipment Lists
(MEL’s) with a letter of authorization issued by a district office.
These constitute a supplemental type certificate for the aircraft and
must be on board. The inspector should compare inoperative equipment
to the MEL to assure compliance. (Refer to Related Task #58, Approve a
Minimum Equipment List.)
F. Airworthiness Certificate. The certificate
most often seen by an inspector is a standard airworthiness
certificate, which is issued for normal, utility, acrobatic, and
transport category aircraft. A restricted, limited, or experimental
certificate must be accompanied by a list of limitations and
conditions (FAR § 21.183 -191) necessary for safe operation. A Special
Flight Permit (Ferry Permit) is issued to aircraft that may not be
airworthy but are capable of safe flight under certain conditions
which are listed and issued with the permit (FAR §§ 21.197 , 91.203
{91.27}, and 91.213 {91.30}). Review the list of limitations and
conditions to assure a valid airworthiness certificate. The N-number
on the certificate must match the N-number on the fuselage to be
valid.

Airworthiness Certificate and Registration Certificate
The standard airworthiness certificate is issued when the airplane is manufactured or when the N number changes. In addition to the standard Airworthiness Certificate, there are Experimental, Restricted, or Special Flight Certificates that may apply to your aircraft.
Registrations must be renewed every three years. The FAA has a page that explains the process. You can also check the status of an aircraft’s registration and who it is registered to on the FAA website.
§91.203 Civil aircraft: Certifications required.
(a) Except as provided in §91.715, no person may operate a civil aircraft unless it has within it the following:
(1) An appropriate and current airworthiness certificate. Each U.S. airworthiness certificate used to comply with this subparagraph (except a special flight permit, a copy of the applicable operations specifications issued under §21.197(c) of this chapter [Special flight permits], …
(2) An effective U.S. registration certificate issued to its owner or, for operation within the United States, the second copy of the Aircraft registration Application as provided for in §47.31(c), a Certificate of Aircraft registration as provided in part 48, or a registration certification issued under the laws of a foreign country.
Radio Station License
A radio station and operators license is required if you make international flights or communicate with foreign stations. As far as I can tell, this requirement is not enforced for flights to Canada, Mexico, and the Caribbean. I got my restricted operators permit in 1980 when they were still required for domestic operations but have not flown internationally.
…you do not need a license to operate a two-way VHF radio, radar, or emergency locator transmitter (ELT) aboard aircraft operating domestically. All other aircraft radio stations must be licensed by the FCC either individually or by fleet. Aircraft operating domestically do not land in a foreign country or communicate via radio with foreign ground stations. Source FAA website.
You must obtain an FCC Aircraft Radio Station License if you make international flights or communicate with foreign stations. If you are not required to obtain a license – you do not need to file this form [Form 605] with the FCC. The license has a term of 10 years. Source FAA website.
At least one person on each aircraft flying or communicating internationally must have a Restricted Radiotelephone Operator Permit. This requirement is in addition to the requirement to have an aircraft radio station license for the aircraft. No Restricted Radiotelephone Operator Permit is required to operate VHF radio equipment on board an aircraft when that aircraft is flown domestically. You may obtain a Restricted Permit using FCC Form 605. No test is required to obtain this permit. The permit when issued will be valid for your lifetime. The fee for a Restricted Permit is in addition to any fee paid for an aircraft license. Source FAA website.
PART 87—AVIATION SERVICES
§87.18 Station license required.
(a) Except as noted in paragraph (b) of this section, stations in the aviation service must be licensed by the FCC either individually or by fleet.
(b) An aircraft station is licensed by rule and does not need an individual license issued by the FCC if the aircraft station is not required by statute, treaty, or agreement to which the United States is signatory to carry a radio, and the aircraft station does not make international flights or communications. Even though an individual license is not required, an aircraft station licensed by rule must be operated in accordance with all applicable operating requirements, procedures, and technical specifications found in this part.
§87.87 Classification of operator licenses and endorsements.
(b) The following licenses are issued by the Commission. International classification, if different from the license name, is given in parentheses. The licenses and their alphanumeric designator are listed in descending order.
(7) RP Restricted Radiotelephone Operator Permit (radiotelephone operator’s restricted certificate)
§87.89 Minimum operator requirements.
(a) A station operator must hold a commercial radio operator license or permit…
Acceptable Radios
As of January 1, 1997, each VHF aircraft radio used on board a U.S. aircraft must be type accepted by the FCC as meeting a 30 parts-per-million (ppm) frequency tolerance (47 C.F.R. § 87.133). The vast majority of aircraft radios that have been type accepted under the 30 ppm frequency tolerance utilize 25 kHz spacing and have 720 or 760 channels. Each aircraft radio has a label with an FCC ID number on the unit. See this post for a short history of radio frequencies. Source FAA website.
Operating Limitations
This part of the acronym seems to generate the most confusion. Per §21.5 aircraft delivered after March 1, 1979 must have an FAA approved flight manual (AFM). Aircraft prior to that date were delivered with an Owner’s Handbook, Pilot’s Operating Handbook, Owner’s Manual, Information Manual or similarly named booklet. These did not have a standard format and the information contained in them varied wildly. They are not required to be in the airplane, however since they give information like landing and takeoff distances—which are required to be calculated for each flight—it would make sense to have them readily available. Many aircraft were sold with an Airplane Flight Manual that listed the operating limitations, required placards, instrument markings, installed equipment, and the weight and balance information when the aircraft left the factory. These are not the same thing as an Approved AFM and there is no regulation requiring that they be in the plane.
If an FAA approved flight manual is required, it is specific to that airplane (Serial number specific.) and is required to be in the aircraft along with any required flight manual supplements.
An FAA approved Flight Manual (AFM) is required to be in the aircraft for all aircraft over 6,000 lbs and for aircraft manufactured after March 1, 1979. In addition, the Type Certificate Data Sheet (TCDS) specifies placards and markings that are required. These placards may include, aircraft category, whether aerobatics or spins are approved, airspeed limitations for flaps and gear, and takeoff and landing checklists. Additional placards may be required by Airworthiness Directives for specific aircraft. One example is AD 79-15-01, requiring a placard next to the fuel gauges detailing the steps required to handle a fuel vapor lock. Some Supplemental Type Certificates (STCs) require additions to the AFM and must be kept in the aircraft. The operating manuals for GPSs, autopilots, and engines usually have this requirement. STCs for things that don’t have an operating component, like exhaust systems or mirrors, do not usually require changes to the AFM.
§91.9 Civil aircraft flight manual, marking, and placard requirements.
(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual, markings, and placards, or as otherwise prescribed by the certificating authority of the country of registry.
(b) No person may operate a U.S.-registered civil aircraft—
(1) For which an Airplane or Rotorcraft Flight Manual is required by §21.5 of this chapter unless there is available in the aircraft a current, approved Airplane or Rotorcraft Flight Manual or the manual provided for in §121.141(b); and
(2) For which an Airplane or Rotorcraft Flight Manual is not required by §21.5 of this chapter, unless there is available in the aircraft a current approved Airplane or Rotorcraft Flight Manual, approved manual material, markings, and placards, or any combination thereof.
(c) No person may operate a U.S.-registered civil aircraft unless that aircraft is identified in accordance with part 45 of this chapter.
The reference it part 45 is regarding the placement and size of the N number.
§21.5 Airplane or Rotorcraft Flight Manual.
(a) With each airplane or rotorcraft not type certificated with an Airplane or Rotorcraft Flight Manual and having no flight time before March 1, 1979, the holder of a type certificate (including amended or supplemental type certificates) or the licensee of a type certificate must make available to the owner at the time of delivery of the aircraft a current approved Airplane or Rotorcraft Flight Manual.
(b) The Airplane or Rotorcraft Flight Manual required by paragraph (a) of this section must contain the following information:
(1) The operating limitations and information required to be furnished in an Airplane or Rotorcraft Flight Manual or in manual material, markings, and placards, by the applicable regulations under which the airplane or rotorcraft was type certificated.
(2) The maximum ambient atmospheric temperature for which engine cooling was demonstrated must be stated in the performance information section of the Flight Manual, if the applicable regulations under which the aircraft was type certificated do not require ambient temperature on engine cooling operating limitations in the Flight Manual.
Weight and Balance
Every textbook I checked says that a document showing the current weight and balance is required to be in the aircraft. None of them cite a source for the requirement. The FARs do not specifically state “a weight and balance report is required to be in the aircraft”. There are two cases where the regulations do require a Weight and Balance in the aircraft. First, if it is specified in the TCDS and second if the aircraft was manufactured after March 1, 1979 and therefore requires an AFM.
The aircraft I fly do not require a weight and balance in the TCDS. The TCDS may contain language like:

Current weight and balance report including list of equipment included
in certificated weight empty, and loading instructions when necessary,
must be in each aircraft at the time of original certification and at
all times thereafter…

If an AFM is required, then the § 25.1583 Operating limitations requires that weight and balance information be included in the AFM.
Neither of my airplanes is required to have a §21.5 “approved Airplane or Rotorcraft Flight Manual” ergo, they are not required to have a W&B in the plane. You could also argue that §23.1589 (a) The weight and location of each item of equipment that can be easily removed, relocated, or replaced and that is installed when the airplane was weighed under the requirement of §23.25. does not require that an updated W&B be included in the AFM only that one must be provided by the manufacturer.
Since the list of things that an FAA inspector is looking for on a ramp check includes a W&B, most people carry it.
§23.1589 Loading information.
The following loading information must be furnished:
(a) The weight and location of each item of equipment that can be easily removed, relocated, or replaced and that is installed when the airplane was weighed under the requirement of §23.25.
(b) Appropriate loading instructions for each possible loading condition between the maximum and minimum weights established under §23.25, to facilitate the center of gravity remaining within the limits established under §23.23.

§135 operations also often require Weight and Balance information—in the form of a load manifest—to be carried aboard the aircraft. See 14 CFR 135.63 (d).

Most of this is straightforward: operating limitations (which may be a handbook or not, depending) are required by 14 CFR 91.9 and the registration and airworthiness certificate are required by 14 CFR 91.203. The radio license is required (or not required) in FCC regulations 47 CFR 87.18 and 47 CFR 87.89.
Weight and balance is not required in any one place. It depends on the certification basis of the airplane. For instance, for a CAR 3 certificated airplane, loading information is in CAR 3.778, "Operating limitations":

(d) Load distribution.

(1) All authorized center of gravity limits shall be
stated. If the available space for loading the airplane is adequately placarded
or so arranged that any reasonable distribution of the useful load listed in weight above will not result in a center of
gravity location outside of the stated
limits, this section need not include any
other information than the statement of
center of gravity limits.
(2) In all other cases this section shall
also include adequate information to indicate satisfactory loading combinations
which will assure maintaining the center
of gravity position within approved
limits.

CAR 3.778 identifies this as an operating limitation, and so is required to be placarded or otherwise carried on board per CAR 3.770 and 14 CFR 91.9(b)(2):

A placard shall be provided in clear view
of the pilot stating: "This airplane must
be operated as a --------- or ---------
category airplane in compliance with the
operating limitations stated in the form
of placards, markings, and manuals."

(2) [No person may operate a U.S.-registered civil aircraft] For which an Airplane or Rotorcraft Flight Manual is not required by § 21.5 of this chapter, unless there is available in the aircraft a current approved Airplane or Rotorcraft Flight Manual, approved manual material, markings, and placards, or any combination thereof

For new aircraft, loading information is part of the AFM per 14 CFR 23.2620.


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Where can I find the information from the back of a TAC if I don't have a physical copy?
The Atlanta TAC has a note that says "See back of this chart for procedural information within the Atlanta Class B Airspace":

If I don't have a physical copy of the TAC - because I'm using a flight planning app, for example - where else can I find the information?

Might be good to explain that TAC refers to 'Terminal Area Chart' (and not 'Tactical Air Command' for example).

If you are using Fore Flight you can Turn On Legends which will show you the legend information for the map. You may also be able to download the information somewhere else in the application. It will help if you inform us of which app you are using.
In the documents section of Fore Flight there are some other Atlanta area documents. Im not sure what the back of the cart looks like so I'm not sure if any of them are correct but there is the Atlanta FLY chart and the Atlanta Class B diagram.

I believe it's the Atlanta Class B Flyaway diagram, not the legend, but I may be wrong.

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How does the stick force and behaviour of aircraft relate to the CG locations?
I'm struggling with developing an intuition for flight behaviour and stick force with CG locations.
Assuming conventional wing and tail configuration, When the CG is moved from aft to forward, the tail will go from producing positive lift, to no lift and to negative lift. The pilot will feel resistance on the stick decreasing to zero and rising again. Since stability is the resistance pilot overcomes to manoeuvre the aircraft, should the stability not decrease and then rise again with the tail loading?

It is a bit more complicated: I found an answer to your question here. https://aviation.stackexchange.com/questions/55885/what-are-stick-position-stability-and-stick-force-stability

"Since stability is the resistance pilot overcomes to manoeuvre the aircraft..."  This is not the definition of stability.

Your definition of stability is wrong. There are different types of Stability. Static stability, Dynamic stability, etc. The most basic, static stability, is the tendency of the aircraft, when disturbed from an equilibrium state, to return to that state on it's own due to the forces (and force-induced moments), to return to that equilibrium state. An aircraft with the CG well aft, (where the tail would be producing positive lift), is generally NOT stable. It would behave like an arrow thrown backwards through the air.

Your question is about the longitudinal static stability of an aeroplane, which is defined as: a disturbance in airflow creates a difference in angle of attack, causing a pitching moment that restores the equilibrium. There are quite a few situations to be considered in an aeroplane with manually operated, reversible flight controls.

Stick Fixed static stability. Described in this answer, containing the image above. And as mentioned in the answers to this question, static longitudinal stability requires that the Centre of Gravity is in front of the Centre of Lift.
Stick Free static stability is the aeroplane creating a restorative pitching moment upon a disturbance when the stick is not held in a fixed position. With manual flight controls, when the stick is released the elevator floats to a position where hinge moment is zero, which differs for every Angle of Attack.
Stick Position stability is described in this answer: when controlling the plane the stick is moved to initiate the manoeuvre, then kept in the same direction.
Stick Force stability is the stick feeling as if loaded by an (aerodynamic) spring: required stick force increases with stick deflection in every stage of flight - take-off, cruise, approach.

Stick fixed static stability is required for Stick Free, Stick Position and Stick Force stability. When stable, the CoG is already in front of the Centre of Lift, so moving the centre of lift forward makes the aeroplane more and more statically stable.

When the CG is moved from aft to forward, the tail will go from producing positive lift, to no lift and to negative lift.

Let's say the CoG moves forward due to fuel in the tail tank being used. The increasing moment arm causes an increasing nose-down moment, which needs to be compensated by a larger aerodynamic nose-up moment:

by trimming the elevator/stabiliser via the trim tab, or
by displacing the stick aft and keeping it there, a fatigueing way of flying...

Both situations need to be continually adjusted as the CoG creeps forward. Trimming the elevator/stabiliser equates to finding a different zero position, and does not directly influence the static stability.

Image above is from prof. Gerlach's uni book (paper copy still in my possession), which shows the aerodynamic moment graphs of the Fokker F27. Strictly speaking, the longitudinal stability is the gradient $C_{m,\alpha}$ at $C_m$=0, the trimmed condition.

The pilot will feel resistance on the stick decreasing to zero and rising again.

They will when not using the trim method to adjust, but the position-and-hold-the-stick method instead. What they feel is just the static force offset to keep the elevator in the position for $C_m$ = 0 though, the force gradient does not substantially change.

Since stability is the resistance pilot overcomes to manoeuvre the aircraft, should the stability not decrease and then rise again with the tail loading?

As mentioned above, stability is the gradient through $C_m$ = 0, not the force offset to keep the stick in an untrimmed position. As can be seen in the F27 graph, gradients of the four lines through $C_m$ = 0 are pretty identical.

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How deep do lift pins go inside the wing in gliders?
In the photograph below, how are the lift pins structurally attached to the ribs and how far does its support structure run inside?

Does the discontinuity in the torsional flow in the wing skins pose any specific problems in the wing design? Also, how much of the torsional load (lets say percentage) is taken by the spar?

One question per post, please.

@OrganicMarble I thought it'd be rather annoying to post three closely related questions.

t's annoying if a person can answer "How deep do lift pins go inside the wing in gliders?" but cannot answer "how far does its support structure run inside?", "Does the discontinuity in the torsional flow in the wing skins pose any specific problems in the wing design? ", or "how much of the torsional load (lets say percentage) is taken by the spar?" When you ask four questions in one post, it breaks the Q&A model of this site, because "a correct answer" can't be posted and accepted.

The lift pins sit in a block of plywood which in turn fills the gap between the rib and the face of the wing skin, as can be seen in the photo. They go almost all the way into that plywood, so they run inside for maybe 3 - 4 cm.  Note that the structure of the lift pin inside the plywood has a larger diameter than the exposed part and is hollow on the inside.
The rib makes sure that the torsional and especially the lift load is well distributed. Since the area encircled by the wing skin is much larger than that of the spar, almost all torsion load is carried by the skin.
Loads go where the stiffness is. In the end, wing skin and spar will bend equally or the wing would disintegrate. Therefore, the strains are the same and the stresses are distributed according to the relative stiffness of the parts.

I had to stare at the pic for a while to figure out what is going on.  The glider has removable outer wing panels with a male/female spar socket and that big retention pin, and the "lift pins" are effectively dowel pins projecting from the inner panel's rib that plug into sockets in the rib in the removable section to transfer torsion loads from the outer panel to the inner panel? What kind of glider is that?  I've never seen wings that break down at that location on a glider before.

@JohnK This is completely normal. I saw this first >30 years ago on an ASW-20 L, and this is common on all wings with more than 17 m wingspan. They need to be divided in three or four parts so they will fit in trailers and the single panels become not too heavy.

Ah that figures.  I've never attended a competition and there weren't any 18m or open class gliders in the clubs I've been in.  I've led a very sheltered life lol.

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Are boundary layer diverters for jet intakes only required for supersonic speeds?
For subsonic aircraft (Mach 0.7), Is boundary layer diverter a necessary requirement for jet intakes? A lot of legacy aircraft were flying without them.

According to the 0th law of aerodynamics: brick flies with enough thrust, no, it's not necessary. But some subsonic jets do have them, e.g. K-8/JL-8 Karakoram, L-39 Albatros.

At the beginning of the jet era, airplanes where powered by turbojet with centrifugal compressor which, as explained here, were able to ingest (and digest) even a not perfectly clean airflow. A typical example of this kind of turbojets was the Rolls-Royce Nene which was used on many jets of the post WW2 era:

With the advent of the more efficient axial-compressor turbojets/turbofans, the cleanliness of the airflow has become important due to sensitivity of the axial-compressor to the airflow it ingests. At that point boundary layer diverters or, more generally, short intakes, have become necessary to provide the first compressor's stage of the jet engine with a clean and stable airflow.

Axial compressor turbojets were used in production aircraft even before centrifugal compressor ones (the BMW 003 and Junkers Jumo 004). As far as I can find all designs that used them had pitot (round, not attached to anything) intakes.

@JanHudec: yes right, they had a straight intake from the nose. It might be that the combination radial jet plus intake without diverted was an intermediate step. I'll add your comment to my answer if you agree.


text:
How are control surface problems in wings having high bending dealt with?
In some aircraft which have large wing deflection due to bending, do ailerons or flaps not get stuck or bent themselves? In the case where hinging axis is also bent how do ailerons operate?

I know aircraft wings are flexible, but that's... Wow...

Part of the answer is visible in the photo--

Here's a few more wings, some bent much more than that.  https://www.popularmechanics.com/flight/g2428/7-airplane-wing-stress-tests/

Re above comment-- re the last example, "Boeing 787 Passes Incredible Wing Flex Test  -- Boeing has completed one of the more spectacular tests of the 787 Dreamliner program. The airplane maker completed the “ultimate-load wing-up bending test” on Sunday using airframe ZY997, the test aircraft that is basically built to be tortured on the ground and never fly. During the test, the wings on the 787 were flexed upward […] "-- if it was never going to fly, why did they finish the fuselage in such detail?  Could make for another ASE question--

And not sure any of these examples are "bent much more than" wings of glider in photo in this question--

@CamilleGoudeseune Those wings cheat - they use hydraulics and electric motors for aileron movement. Glider wings do it properly, with pushrods between aileron and stick.

@quietflyer, the airframe is used for a whole lot of different tests, so it makes sense to build a complete one.

Flaps and ailerons are weak in bending and have multiple hinges along the span. Also, as you can see in the photo, the flaps are segmented which not only helps to optimize the deflection angle over span but also limits any sideways offset of the hinge points due to bending. Using only glassfiber or aramid fiber for the flaps makes them very flexible so they easily follow the bending of the wing.
Flexible flaps are also torsionally flexible. This necessitates to distribute the mass balance along the span which is normally achieved by glueing a brass rod to the forward edge of the flap which reaches into the flap cutout of the wing.
The control surfaces are connected with pushrods. Those are segmented as well and have ball joint swivel bearings between them to allow for some misalignment. The joint is held in place by a swiveling buckling support; in the sketch below (own work) it is attached with its hinge point at the lower wing skin; it could as well be placed horizontally in the wing and be supported at the spar web.

If you follow the deformation of the wing, you will see that in this example the pushrods sit near the upper surface of the wing. This will make them relatively longer when the wing structure deforms in upward bending, and will create a negative aileron deflection (trailing edge up), which reduces lift on the outer wing and reduces bending. The linkage can, therefore, be used to limit loads and dampen the bending motion.
The design has to account for the maximum possible elastic deformation and for differences in thermal expansion between structure and linkage.

Thank you for answering and brilliant sketches sir. Another question I have now is when the hinge axis is curved does it cause problems? I think aileron travel should be affected when the whole wing is curved like that

@Mridul I expanded the answer.


text:
How do ultralight helicopter like mosquito manage to fly without a swash plate?
It doesn't seem to have a swash plate. Does pilot shift his weight like in a hang glider?

Pretty sure Mosquito has collective and cyclic -- it hovers well.

This is the swashplate, within the mast, at the level of the black belt

Why would someone downvote this?

The pictured helicopter is a Mosquito Air. It does have a swash plate, it's just not visible because it's inside the main mast. The swash plate is connected to the blades via pushrods in the hollow rotor shaft, which connect to the blades from above.

I be interested in building one if it had a more reliable engine.

No @JohnK, just no

Hey I'm adventurous, as long as it's 4 stroke or turbine lol.

@JohnK There actually are turbine-powered Mosquitoes out there.

Yeah I know.  Too expensive.  It needs a 4 stroke that can match the p/w of the Rotax.

@johnK 600cc engines salvaged from superbikes?


text:
Is all the extra lift from flaps added upstream where the spars sit?
Flaps can significantly increase the lift for a wing, is all that extra lift added upstream where the spars sit? I ask this question because the flaps in airliners seem to be only connected by a few links and actuators.

Source: dreamstime.com

It seems like the title question and the accepted answer are not in agreement. The accepted answer pulls from the body of the question. This results in a confusing situation for readers. @Mridul, would you mind editing the title so that it reflects the question in the body?

I see what you mean.  90% of the answer is about bending moment, which is arguably not relevant to how the question reads now or was originally asked.  The bottom 10% of it answers the question as it reads now.  Only the drive links are clearly visible in the photo; the strong support and coupler frames, etc., aren't clearly visible, but they're inside the partly-orange structures under the flaps


text:
Can the torsional strength of a wing be increased by adding more ribs?
I understand increasing the skin thickness is the best way to increase the strength in torsion, but if that is not an option will just adding more ribs work?
In a composite wing skin, will adding a core between the skins increase its torsional strength and rigidity?

It will not help if your failure mode is the shear strength of the wing skin being exceeded, because the moment of torsion will act everywhere along the span, also between the ribs. In this case you will have to make the skin thicker or increase the cross section of the wing to reduce shear stresses. In this case, making the foam core of a composite skin thicker will also not help, because the core does not carry any (or barely any) load
If your failure mode is buckling of a section between ribs, then adding ribs will help, because they would make those sections smaller and more resistant to buckling. Increasing the core thickness of a composite skin will also help against buckling.


text:
Is constraint diagram an useful tool for unmanned aircraft design?
I've come across constraint sizing diagram methods for preliminary but its for general aviation aircraft. Are the general equations used for drawing performance curves valid for unmanned aircraft design which weigh 50-150kg?

Community, please see Drone/UAV Question FAQ and decide if you would like to answer this or migrate (Upvote this comment for migrate)

What would lead you to believe it might not be applicable?

Allow me to give an opposing opinion, it could be answered here, the physics are the same. Up to 150 kg? Could be a manned aircraft. No need to migrate this one away I would say.

That sizing plot can definitely be used also for UAV. Obviously you have to use the relevant requirements: for example, the blue line translating the takeoff length, most probably derives from some FAR or MIL rules which might not be suitable for UAV.

Dear downvoter,  could you please explain how this answer can be improved?

Ok, I'll bite. I downvoted this answer as the tooltip says "It is not useful..." in any meaningful way. It's too short, it lacks any detail and most of all, it does not answer the question. The question was "Are the general equations used for drawing performance curves valid for unmanned aircraft design" which was not addressed. TBH, the question was not a good one, and therefore at that point you're much better off seeking improvement from the question asker than writing a substandard answer to a substandard question.

We're after quality questions and answers here, answering substandard questions just fuels the fire of substandard questions.

@Jamiec: ok, thanks for your explanation. In my opinion the question isn't that bad otherwise I (or anyone else) couldn't have answered and it would have been massive downvoted or even closed


text:
How conductive is Carbon fibre in aircraft skins?
I ask this question in relation to radar cross section returns, Are the EM reflection properties of carbon epoxy composites similar to aluminum? Also how well are EM shielding characteristics of CF composites?

Somewhat related: How well does a composite fuselage deal with lightning?

Structural carbon fiber reinforced plastic, as used in aircraft fuselages and wings, is a radar reflector.
CFRP's RCS properties are similar to aluminum, except for unidirectional fiber, if its fibers are oriented parallel to the incoming signal's polarization. In the graph below, CF 1-2 are twill weave and CF 3-4 are unidirectional.

Source: https://www.researchgate.net/publication/291698935_Characterization_of_Radar_Cross_Section_of_Carbon_Fiber_Composite_Materials
7-12 GHz corresponds to the radar X-band, in which most military radars operate. A difference of a few dB for a narrow range of signals doesn't provide much concealment, so CFRP aircraft aren't inherently stealthy.
There are non-structural radar-absorbent materials that use carbon fiber chopped to specific length. They aren't strong enough to build high-performance airplanes out of, but can be applied on top of a structural CFRP layup, or embedded into a fiberglass layup.
One paper shows up to -18 dB peak reduction in RCS with 3.2% of short CF added to fiberglass, but that's in a narrow band. A different paper shows a more broadband -10 dB reduction for CF addition.

In general, CF must be combined with other materials, such as iron oxide, to provide comprehensive reduction.
A comprehensive review of carbon-based RAM shows that -10 dB reduction across the board is achievable with one material, but pushing it to -20 dB requires a mixture of different materials, in a layer about 2 mm thick. This has to be on top of the reflective structural skin.

Electromagnetic shielding uses the same conductive properties, but unidirectional fiber isn't a good fit for shielding, since it lets some signal through. Generally CFRP needs to be supplanted with metal mesh or a mat of chopped fiber to provide metal-like shielding.


text:
Should the planform area inside the fuselage be considered for a stability calculation?
I am using lower order tools (xflr) for calculation of neutral point and CG location.I can think of modelling the wing in three ways

wing with section inside the fuse missing

wing with section inside the wings modelled

section insde the wings removed and the outer wings connected.

What should be the best way to calculate the neutral point and CG?

For defining the wing area?

See here

If you are literally modeling these choices in XFLR5, they will not only affect the value of Sref (which doesn't really matter, so long as you are consistent) -- but they will also affect the lift distribution.  1) will add erroneous wing tips near the aircraft center -- forcing circulation and lift to drop to zero there.  3) will move the wing tips inboard, reducing the span.  2) will do the best job of modeling the aerodynamics of this system.

At least in pre-design phase, wing is considered as being one single surface extending/prolonging inside the fuselage and the surface "inside" the fuselage counts as wing surface, just a convention.

@RobMcDonald Vortex are shed whenever there's a spanwise change in lift distribution. Since at root of a real aircraft the lift will reduce to zero as we approach the fuselage (correct me If I am wrong), large vortex should be shed here. Doesn't case 1 simulate that more closely?

@Mridul Yes there is some dip in the lift distribution near the fuselage, there is usually some amount of lift carryover.  Capturing this perfectly is surprisingly difficult -- either computationally or experimentally.  The dropoff with a fuselage is generally not as severe as with a gap between the wings.


text:
What are the effects of having the aileron hinge line on the pressure side vs the suction side of a wing?
I have to choose where to place the hinge line of the aileron of a large remote control aircraft which is to be optimised for endurance. Which is the most optimal placement of the hinge line of ailerons?  Consequently, What will be effects of having the linkage shroud on top surface vs having it on the bottom surface?

Here's one thing large endurance aircraft can do: they use spoilerons instead of ailerons and rudders.
You simply leave off the downward pointing off-side aileron, which creates adverse yaw.
The inside one will create "proverse yaw".  If you design it right, coordinating rudder will not be needed to turn.
Generally, bumps and irregularities are best placed underneath the wing, in the region of slower air and higher pressure.  Any disruptions of upper wing airflow can be very detrimental to lift.
As a designer, another quandary would be trying to avoid holding ailerons in the turn. Planes with strong dihedral effect have this tendency.  Avoiding swept wings would help here.
Of course, the best way to maximize range is not to turn for long periods of time in the first place.

The upper part of the wing is the one "doing the job" of keeping the airplane airborne and the airflow there should be left as much as possible undisturbed, especially toward the trailing edge. Anything there could lead to the detachment of the boundary layer and the subsequent stall of the wing. If you can choose where to locate the shroud, then on the belly of the wing is better.

@jkztd I think he is referring to the hinge shrouds on the belly

The top picture shows an aileron with a center-line hinge line.  You can tell because the circular leading edge of the control surface will sweep out an arc maintaining the upper surface seal -- the hinge line will be at the center of that arc.  The bump on top is the pushrod of some sort.
The bottom picture shows a flap with an approximated Fowler action.  It can deflect down only, with the hinge line far below the bottom surface of the wing.
Usually ailerons deflect symmetrically and usually have their hinge lines on the center line.  I am sure there are exceptions -- most likely for construction simplicity.


text:
Is the classical definition of induced drag merely a mathematical construct?
I understand that the presence of vortices at the wing tip cause part of wing to see a higher angle of attack and cause more drag. But the explanation that lift vector tilts backwards seems like a mathematical construct to me since I defined lift as force along a vector of my choosing in the first place. Isn't it using the effect, namely reduction in lift and increase in drag as a cause to explain the phenomena?
I do not contradict the existence of induced drag, I just want to come up with a better explanation for the phenomena.

I consider the best explanation to be the necessity to satisfy conservation of energy and conservation of momentum, because that does not depend on the mechanics of how lift is being generated as long as it is aerodynamic.

Isn't the "of my own choosing" part just as much an artificial construct as induced drag is? The force vector coming from the wing can be seen as two vectors (lift + induced drag), one vector (lift, tilted backwards), or whatever 2+ vectors you like that sum up to the one. To the extent that induced drag varies with speed & is a component of the total U-shaped drag curve, it seems to be a useful construct for understanding what wings do.

I've always thought of induced drag as a representation of the energy consumed in redirecting the package of air the wing influences, in order to generate the pressure gradients and flow direction changes that make up the lift force.  In other words, the energy consumed to induce the air to move down.

You are right, the definition of lift implies that its direction is given, so its vector cannot tilt backwards. A better expression would be "backward tilt of the aerodynamic force vector", although this is a bit longer.

Related: https://aviation.stackexchange.com/questions/94469/induced-drag-where-does-it-actually-arise-from/94470#94470

All of physics is artificial construct. Nothing of it is how the world actually works but rather our description of patterns we found in it. We do it using terms that exist only in our minds. So lift and drag, being terms, are artificial constructs.
Now there is a pattern we always see in the world that we describe as conservation of momentum and conservation of energy. Using this we can conclude that any aerodynamic device (usually airfoil, but applies equally to e.g. Flettner rotors) that creates a force perpendicular to the flow (which we call lift) also creates a force in the direction of flow (which we call induced drag).
To create the lateral force, force has to be applied to the air (via principle of action and reaction), which accelerates it sideways (downward, usually), changing its velocity. But:

In the reference frame of the device there is nothing to increase kinetic energy of the air, so the magnitude of the stream velocity can't change, so to add sideways component the component along free flow direction must reduce and that requires forward (in flight direction) force on the air, which causes aft reaction on the aerodynamic device.
In the reference frame of the free stream the air velocity does increase, which means the aerodynamic device must do work on it, and since work is dot product of force and path, the aerodynamic device must be applying force in the direction of its motion, and have force in the opposite direction applied by the air.

From this argument it also follows that this component of drag decreases (for given lift) with:

Square of flow velocity as more air mass is available, so it only needs to be deflected by less, and since momentum grows with velocity, but kinetic energy with square of velocity, the work that needs to be done for the same lift decreases.
Cross-section of the affected flow, which grows approximately with square of span, for the same reason.

That should explain how the laws of physics we observe make those specific definitions of lift and drag work well. But it's still in our minds, not in the outside world. The outside world does not care how we call things.

This question is as much philosophical as anything else. All our mathematical models are only approximations which predict what will happen in certain circumstances. They do not necessarily address what is "really" going on.
To take a common example from another field of engineering, is a given sound a particular waveform or a particular spectrum of frequencies? It is produced by some vibrating object and moves through the air as a wave, yet is detected by the hairs of the inner ear as a spectrum of frequencies. Mathematically we move between the two descriptions, the time and frequency domains, via the Fourier transform. Digitally, we quite often convert between the two as it suits. But which is the "real" nature of sound? Ultimately, that question is meaningless, its reality is what it is and we just look at it in the most useful way at the time.
The same can be said of induced drag. We can say that the lift vector is tilted backwards, or we can say that the induced lift is vertical and the induced drag is horizontal. There is no right or wrong, there is only utility for the current calculation.

All of the force things we talk about in aviation are mathematical constructs. All Aerodynamic forces, Lift, Drag, Induced or parasite, on whatever vector direction you choose, are constructs. The only real forces that an aircraft (or any body in a fluid) feels, are the forces exerted by the little molecules bouncing off the surface of the body. Each time a molecule hits the surface, and bounces off, there is a change in momentum, and a corresponding force exerted on the surface normal (perpendicular) to the surface, at that specific point where the molecule struck. All the constructs we talk about are just integral summations of all these tiny forces which are actually distributed over the entire airframe. We summarize them and pretend that an equivalent single force exists, which acting through a single point on the airframe (the aerodynamic center or AC), and chosen so that the actual effect of this constructed Psuedo force will be equivalent to the actual effect of the uncountable individual forces of all the molecular collisions discussed above.
Lift, Drag, etc. are just engineering psuedo forces chosen to add clarity or to allow engineering to perform calculations without having to deal with all the individual molecular forces, and get the same (or equivalent) results. Generally, Lift is chosen as the component of the actual aerodynamic Force which is normal or perpendicular to the aircraft flight path vector, and Drag is that component that lies parallel to the aircraft flight path vector.
Even the thrust of a propeller is in actuality, caused by the collision between air molecules and the cambered surface of the propeller, in the exact same way. Same with the thrust of a jet engine. It is caused by collisions of the high pressure combustion gasses on the turbine blades inside the engine. All the other formulae and shortcuts we use (like Bernoulli principle, or application of downwash comparisons for lift explanations), are accurate, not because they represent what is really going on, but because they take advantage of principles of physics like energy  conservation of or momentum conservation.

I wouldn't call Lift a 'pseudo' force. This term is reserved for 'inertial forces', whereas lift and drag are components of a pretty much 'real' aerodynamic force.

@Zeus, Thanks for that. Yes, in the Physics community, that word has a well-defined meaning. But the force they are components of is a integral summation of an uncountable number of "real" forces, so it is itself, a mathematical construct. I did not intend to imply by use of the word Psuedo any specific well-defined meaning as is used in physics for e.g., Centripetal forces. I just meant that they are constructs, not the real forces.  pretty much real ??!! I don't remember that phrase from my physics courses !


text:
What do these terms related to airfoils mean?
I keep coming across some terms related to airfoils and I'm not able to find clear definitions for these terms.

Bubble ramps
pressure recovery
suction side

@mins I understand how a bubble is formed, but what is it that is referred to as a ramp?

A ramp is a mild positive pressure gradient which destabilizes the boundary layer so the bubble will develop. Without a ramp, bubble location on the suction side will vary  a lot over angle of attack. When transition has happened, the steep pressure recovery may follow (but not sooner).

Thanks, are transition ramp and bubble ramps the same thing?

Whether a bubble forms depends on the Reynolds number, but essentially yes.


text:
How to reverse course when there is no published course reversal?
Coming from the SW on the RNAV RWY 15 KONO, how do I align myself with the final approach course?  There is only one IAF and no published course reversal.  Is it up to me how I get established inbound?
[RNAV 15]

If the approach doesn't contain a published course reversal (ie PT barb or Hold-In-Lieu), the approach must be flown straight in. In this case, you will either get vectors-to-final or you will get cleared via one of the feeder routes.

Specifically, ATC will not (cannot?) clear you for a straight-in approach that requires a turn >90° at the IAF; this is why so many RNAV IAPs have a "Basic T". This one doesn't, but PIYID and NEYGI serve the same general purpose, and being on V269 makes them easy to file to.

Right...that was the logic behind me discounting ATC clearing the aircraft Direct CEYTA.

It's a RNAV approach. You just fly to PIYID waypoint and that's all. I think all those reversals very useful when there was no RNAV and there was no other reliable way to establish on the final approach course unless you had VOR's and NDB's not at the airport to assist with that. Now it does not make sense. Why to fly more miles if you can fly this approach as published directly.
There are 3 real life situations:

If you fly commercially you will have a flight plan that will end at PIYID.
If you just fly somewhere and ask tower for the approach you will get direct to PIYID.
If it is uncontrolled airport probably it will make sense to go directly to PIYID... or other waypoint.

"Why fly more miles"...  Your recommendation to PIYID then CEYTA essentially creates a 9.9 nm base leg.  I would prefer to reduce that.

"Approach, (callsign) request 3 mile offset to the west at CEYTA for alignment."
The offset you request will be what is necessary to execute a roughly 180 deg turn to the inbound course.  Keep it snug, but don't cheat yourself out of the space needed to make a comfortable turn in IMC with a little bit of wings level time before you hit the fix.
ATC knows you can't get there and pivot in place, and are usually very accommodating if it is clear what you are asking.

Assuming you fly in the US, ATC will not give you this clearance! They will either give you vectors-to-final, or they will give you direct PIYID for the feeder. They aren't allowed to give turns of more than 30 degrees to intercept the final approach course, nor are you allowed as the pilot to perform your own unpublished course reversal.

Not true unless this is a recent change.  I haven't flown IFR in 13 years, but I flew continuously for the 16 years prior to that and I have asked for and received this many times.

... in the United States.


text:
Why is the weather radar mounted on the left wing of the TBM?
Weather radar is mounted starboard on the Pilatus PC12 or in the Piper Meridian/M600/M500.
It adds drag to the right wing, thus a right yawing moment, and helps to counteract the slipstream effect and P-factor that tends to yaw the aircraft to the left.
So why is the wx-radar mounted port on the TBM ? (the propeller rotates in the same direction as the PC12, so it seems like the wx-radar only adds more yaw to the left)

Because it's French?

@JuanJimenez: "Because it's French?", hum those guys have pioneered aviation, to the point aviation is a French word.

@mins Aviation was invented before the French even knew they existed as a people, but I can see the French have yet to invent comedy.

I sent a query to TBM to try to find out.

I sent a query to the sales department at TBM as promised, but they never answered.

Another context for radar pod ( probably wouldn't be described as "weather radar" ) and TBM : https://www.tapatalk.com/groups/hyperscale/am-1-48-avenger-radar-pod-question-which-is-best-d-t204685.html

Perhaps to have less wiring distance from the unit to the display in the cockpit? Any ideas on the cockpit layout for the 2 craft?

After some detective work I have concluded:
In the TBM planes, the center portion of the right wing leading edge is occupied by the stall warning vane (or an optional AOA sensor in newer models). The two pitot tubes are located under each wing and landing lights are positioned at wingtips. This would leave the current location for the optional weather radar pretty much the only option available.

TBM 700C pilot information manual, stall warning vane description on page 7.15.1
Source for picture


text:
How many flying mileage a pilot should have per year in order to keep his licence valid?
Please let me know if possible
Thank you very much

There is no mileage requirement.  For info on how to maintain a license:  https://aviation.stackexchange.com/questions/758/what-does-it-take-to-maintain-a-private-pilot-certificate

In the US, licenses don't expire, so they are always "valid". Whether you can fly or not (in the US) just depends on currency requirements. If you are flying by yourself, all you might need is a biennial flight review (one flight every 2 years, if you can pass it).

@Ron Beyer: And the appropriate medical, if required.

Welcome to aviation.SE! Can you tell us which country or regulations you're asking about? The requirements can be different everywhere, e.g. see this question for the US, which is quite different from many other countries. A very general answer to your question is that pilots care about time flown, not distance. If you're new to StackExchange you might find the [tour] is useful.

Any requirements would be in the form of flying hours, or takeoff & landings, not miles.  (Different airplanes fly at very different speeds.)  For instance, in order to carry passengers, a private pilot must have performed 3 takeoffs & landings in the preceding 90 days.  (14 CFR § 61.57)

There’s no minimum flight time required to retain a pilot certificate, though there are minimum currency standards in order to act as PIC in certain flight situations, or to carry passengers.  See §61.57.
Pilots are also required to complete a Bienneal Flight Review ever 24 calendar months in order to act as PIC aboard an aircraft.  Any successful practical exam for an airman certificate additional category, class or type rating can be substituted for a BFR.
Typ ratings listed on airman certificates expire after 12 calendar months unless the applicant undergoes a recurrent training for the aircraft type and pass a practical exam administered be an aviation examiner or check airman.
Flight instructor certificates do expire after 24 calendar months, unless the CFI:

Passes a flight instructor refresher course containing both ground and flight training within the preceding 3 calendar months on the expiration date.
Pass a practice exam for one of the ratings listed on their instructor certificate
Recommends at least 5 students to take a practical examination in which 80% of whom pass, or
Work as a chief pilot or check airman for a Part 121 or 135 operation.

See §61.197


text:
What's the difference between multiple disc brake and segmented rotor brakes?
Going through FAA's aircraft maintenance manual I came across a topic under braking systems, which had two different types of brakes: multiple discs and segmented rotor disc brakes
Looking at the their construction:
Multiple discs brake

Segmented rotor brakes

These look pretty much of same construction.
Also, reading through the description, I couldn't find much information apart from just that "Segmented rotor disc brakes are used on even higher performance modern aircraft"
Aircraft brakes
The information provided in the above-attached link is the same information as printed in the FAA AMT Handbook VOL 2
How are they any different? Or am I missing something here?

More properly, both types are multiple-disc brakes. Disc brakes, in turn, can be either continuous or segmented.
Regular continuous disc brakes are somewhat similar to car disc brakes, except for the brake pads being replaced by an entire stator disc. The calipers can be placed in one spot, like on a car, or on larger planes spaced symmetrically around the disc, and the areas away from the calipers are only pressed against the rotor by the pressure plate's and the stators' rigidity.
Much of this action is lost as the discs heat up and expand, causing them to warp. When the discs are warped, only the closest part of the disc keeps making contact. The pressure is very inconsistent among the disc, governed by geometry; something will touch, but it's a few radial lines across the disc, not the whole.
Segmented disc brakes return to (semi-)individual brake pads, spaced around the disc evenly, with a caliper per each brake pad. Most importantly, the rotors are split into separate sectors (only loosely connected around the carrier), so they get room to expand in all directions.
When a segmented disc expands from the heat, the sectors almost touch, but without significant warping. And each brake pad is forced against the rotor by its own caliper (even though there's a pressure plate in between); they are less fixed geometry and more fixed pressure. The theoretical contact area is smaller overall, but it's maintained constant even as the brakes heat up. All of the brake pads keep creating friction against the rotors at all times.
I could probably sketch something up, but you can as well do an experiment - push two plates of any material firmly against each other with your hands and rotate them against one another. It won't be difficult to do, even if they're rough, because your hands aren't very rigid, not ensuring constant contact areas. Now place a few clothespins around their perimeter. You'll notice that they'll become much harder to turn against one another, even though the clothespins are weaker than your arms.
Regular disc or multi-disc brakes clamp only a small part of the disc, with rigidity somewhat helping elsewhere, like with two plates held in your hands. Segmented-disc brakes rely entirely on the latter effect, braking at multiple very firmly clamped areas across the disc stack. They provide strong braking torque even when very hot.


text:
How does an outflow valve work mechanically?
Under Environmental control and oxygen systems chapter of Aircraft Maintenance and Repair book by M. Kroes, there's an topic of working of the outflow valve

The diagram is of a valve that also works an altitude limiter and positive pressure relief valve. What I don't understand is the working of this valve from the diagram.
The text explanation states, when cabin pressure reduces below a preset value the bellow expands and it moves the poppet and introduces reference chamber air in the case, effectively closing the outflow valve. How is the outflow valve connected tp the bellow chamber?

If the cabin altitude
exceeds the preset limit, the bellows expands enough to press
on the small poppet just to the right of the bellows. This opens
the reference chamber to cabin air pressure and causes the outflow valve to close. The valve remains closed as long as the
cabin altitude is above the setting of the bellows. This operation
can be overridden by the positive pressure-relief control system

Above is the explanation from text.
Also, why are outflow valve openings so big?
And, do all the pressurised sections of the aircraft have separate outflow, or are they connected to a single unit via lines from the pressurised sections?

Outflow valves and pressurization differ tremendously from aircraft to aircraft. The aircraft that I have flown have one pressure vessel and one outflow valve. There might also be a pressure relief valve in case the outflow valve gets stuck in the closed position, and a negative pressure relief valve.

The driving force for the valve is a vacuum source that is connected at the vacuum connection at the bottom of the diagram.  The valve operates by varying suction to chamber 3, creating a pressure differential between 3 and 1 (cabin pressure) that allows the pressure in 1 to drive the valve open (to the left in the diagram) against the return spring.  Take away the vacuum, and the spring will close the valve (move to the right).
The vacuum source is usually an ejector pump in the engine bleed air plumbing, so the vacuum is available whenever the bleed system is on.
So in other words, the valve is sucked open (by bleed-powered vacuum), and sprung closed, and is modulated by varying the suction (by the electropneumatic control valve) against the constant spring pressure.  More suction moves in more open, and less suction moves more closed, and if the suction is completely removed the spring fully closes the valve.
On transport aircraft there are normally two outflow valves for redundancy, but both will usually be located next to each other, typically at the rear pressure bulkhead, and all of the air in the pressure hull will exit at that spot, all of the pressurized zones being interconnected.  Where separated spaces are vented to each other in the pressure hull, but would have a hard time equalizing quickly in a rapid decompression event, like a cockpit bulkhead with the door closed, there will be special blowout panels (like in the cockpit door or elsewhere on the bulkhead) to allow equalization to occur rapidly.
You will also typically find a separate dump valve that opens on on the ground to keep the aircraft from being (slightly) pressurized by ground cart inflow when the bleeds are off and the outflow valves are closed (which can cause problems with opening plug doors), and also to provide an exhaust path for avionics cooling during ground operation with the main doors closed.
The opening size of the pressurization outflow valve has to be large enough to allow more air out than the maximum bleed flow in, so it has to have a fairly large exhaust opening to be able to control pressure at the margins with the pressure hull as airtight as it can be.
Aside from altitude, how much the valve opens in normal flight depends on the main air conditioning bleed inflow (which is usually constant, being regulated bleed flow) and how leaky the hull is (an older airplane with worn door seals, leaky rivets etc, can be quite leaky, and the valve may run pretty much almost fully closed some of the time because of all the air that is getting out through other points - if you've ever been at the forward galley area and wanted to hold your ears due to some crazy howling sound, that's a door leak, and the outflow valve will be running more closed than normal to compensate).

The pressurization outflow valve picture you have in your question is one used on quite a few aircraft.  In chamber 3, there is a spring that pushes the outflow valve closed.  Walking out to an aircraft without the engines running, we can be assured the outflow is spring-loaded closed.
On the bottom left of chamber 3 is a vacuum connection.  That vacuum connection is connected to a vacuum pump or ejector pump.  It is typically regulated to a certain PSI lower than cabin pressure.  On the jets I fly, the "vacuum" control pressure is regulated to 4.5 PSI lower than cabin pressure.  We are not talking about absolute zero pressure but a PSI lower than cabin altitude.  As an example, sea level pressure is roughly 14.7 PSI.  If the cabin was at sea level, the control pressure would be 10.2 PSI.  That difference of 4.5 PSI is sufficient to pull against the spring and open the outflow valve allowing cabin  air to rush out around it to the outside.
Section 1 uses cabin pressure and compares it to outside air pressure.  If the cabin altitude is ever higher (lower pressure) than outside altitude (higher pressure) that will in turn cause a suction and the outflow valve will open.  It ensures the cabin will not have a negative cabin differential.
Cabin differential is simply the difference between cabin pressure (in PSI) and outside air pressure (in PSI).  You may remember that air pressure is cut in half approximately every 18,000 FT.  So for an example, if the cabin altitude is sea level or 14.7 PSI and the aircraft was cruising at 18,000 FT MSL or 7.35 PSI.  The differential would be the difference 14.7 - 7.35 = 7.35 PSID (PSI differential).
Section 2 appears to be an over-pressurization relief valve.  If the cabin differential is too great, the diaphragm and spring will actuate causing the outflow to open to raise cabin altitude (or lower cabin PSI).  This in turn will lower differential pressure so the pressure vessel doesn't explode.
There are two things that are generally required to pressurize an aircraft.  The regulation of airflow coming into the cabin.  This is typically accomplished through the pneumatic and air conditioning systems and the regulation of airflow exiting the cabin which is accomplished through the pressurization system.


text:
Why is more oxygen required at higher altitudes?
Referring to a chapter from Aircraft Maintenance And Repair

Normal air contains approximately
21 percent oxygen, and this provides adequate oxygen for
the human body at lower altitudes. At 34 000 ft [10 363 m]
altitude, a person must be breathing 100 percent oxygen
to absorb the same amount of oxygen as when breathing
air at sea level.>

Why is it required for us breathe 100% oxygen to absorb same 21 percent of oxygen we would breathe at ground/ lower altitudes?

Related: Why is the time of useful consciousness only seconds at high altitudes, when I can hold my breath much longer at ground level?

The answer is you don't need more oxygen. You need exactly the same AMOUNT of oxygen. But the density of the atmosphere decreases as you climb to higher altitude. (the density is created by the weight of the air above you). So the AMOUNT of oxygen in each cubic inch of atmosphere (or in each breath you take) decreases. To compensate for that, to keep the AMOUNT of oxygen in each breath the same, you must increase the percentage of each breath that is oxygen. So at Sea Level, the pressure is 14 lbs per sq inch, and the atmosphere is 20% oxygen. To get the same AMOUNT of oxygen at 17000 feet up, where the pressure is only 7 lbs per square inch, you would need to be breathing air that 40% oxygen. And at an altitude where the pressure was only 2.8 lbs per square inch, (this is somewhere around 38,000 feet as I recall), you would need to be breathing 100% oxygen.

So in a way, we're just substituting the lost atmospheric pressure with oxygen?

The other gentleman's answer speaks in terms of what is called the Partial Pressure of oxygen (20% of 14.7 lbs per sq inch (psi) = 2.8 psi, So, yes, you could phrase it that way, although I would not use the concept of substitution. You just need to have enough oxygen in each breath you take. You need 2.8 psi of Oxygen to be equivalent to sea level.... But people can acclimate, some live and survive in the Andes at 16000, 17,000 feet, where the partial pressure of oxygen is only 1.4-1.5 psi....

That just completes the whole answer, thanks!

This question is best answered on https://biology.stackexchange.com, but in short it is the partial pressure that counts. This is (pressure in atmospheres) * (percentage).
At 0.5 atm (18 kfeet) you need twice the amount of oxygen in the air (aka 40%) for it to have the same effect on you. Above 38.5 kfeet the pressure is 0.2 atm, so even 100% oxygen will only provide 20% partial pressure of oxygen, which is the same as normal air at sea level

There's a good table on this at https://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html.

The answer is you DON'T. You need exactly the same AMOUNT of oxygen. But the density of the atmosphere decreases as you climb to higher altitudes, so the AMOUNT of oxygen in each breath decreases. To compensate for that, to keep the AMOUNT of oxygen in each breath the same, you must increase the percentage of each breath that is oxygen...


text:
How is true airspeed measured?
Now, indicated airspeed is pretty much the dynamic pressure that air is creating.
What about true airspeed? How do the calculations go about factoring out all the wind patterns and finding the speed?
Also, for ground speed, how can we factor in the head, tail and crosswinds to determine the ground speed?

You can convert between IAS and TAS using the formulas e.g. in this answer. Also see How is the wind direction and wind speed measured in flight?.

You don't measure TAS. You measure IAS (or CAS), then correct for altitude and nonstandard temperature to get TAS. As a rough rule of thumb, add 2% to the IAS per 1000ft of density altitude.
With modern avionics, you know your GS from GPS or similar systems, so the difference between TAS and GS tells you the head/tailwind component and course vs heading tells you the crosswind component, which will be combined to display the direction and speed of the actual (not forecast) wind.
With older (or no) avionics, you get a winds aloft forecast before departing, and you do the reverse of those same calculations with an E6B to find out what heading to fly to get the desired course.


text:
Do aircraft usually fly with CG at 50 percent of chord or do I need to reduce the tail volume?
I'm working on a design having a 3.2-metre wing with a constant chord section in the middle and taper on the sides. With a tail volume of about 0.68, the neutral point is at 60% chord from the leading edge. I want to choose a static margin of 12.5, which puts the CG at the middle of the chord at the midsection. Also, the main spar is twice farther from CG than the false spar in this configuration which should put more load on false spar than main spar. Bringing CG to 33 percent of chord requires a TV of 0.4 which I think is considerably less from what other aircraft in the same class use.
My question is - Do aircraft usually fly with CG at 50 percent of chord and would it be a good idea to reduce the TV to less than 0.4 to bring CG where I want? to reduce TV, should I keep the same tail area and reduce the length of boom? Also, should I change the spar locations to suit the CG location?

Add some weight to nose if you can.

Tail volume has very little to do with CG, and it is rather silly to have it so far back that the tail has to lift.
Unless there is a specific reason to put CG at 50% chord, put it at 25% for starters.  You can always move the CG.
Tail volume (specifically horizontal) is there as needed not only for trim in flight, but also for directional stability and stall control.
It needs to be big enough for that too.  Also keep in mind that weight further forward increases the leverage of the tail, allowing for smaller size.
Now the "smallness" will be more related to that which is needed for pitch control.
It may be best to reference aircraft of your dimensions and weight to improve your design.  If in doubt, leave the TV a bit large, as this will result in a very slight increase in fuel consumption.  This is generally not a problem for models of that scale, and makes the plane safer to fly.
Only airliners that use millions of gallons of fuel a year need to "cut it" any finer.

I'm confused because I have been reading articles that say tail volume affects neutral point which should affect the cg location.

@Mridul  The wing also helps with stability if CG is further forward (as does any fuselage aft).  There is very little reason to have CG further back.  Why not put it near the main spar?  So, you see, there are 2 ways to handle NP:  weight forward or bigger tail.  Which will you choose?

Don't confuse NP with net CP (center of G and Lift).  NP is further back for stability.

I calculated neutral point of the aircraft in Xflr and used the static margin of 12 to get a CG location. If I put CG at 25 percent chord, will it not make the aircraft too nose heavy?

@Mridul Maybe 33% will be better, but it will help if it is slightly ahead of wing CP, requiring a little downforce from tail.  You can move NP back a little (if need be) with a boxy fuselage aft, but most planes with tails and forward CG are adequate in directional stability.

NP is the aerodynamic center of the airplane's whole horizontal footprint.  Static stability is basically a weathervane effect about the C of G, being the pivot, and the NP being the aerodynamic center of the weathervane.  Trim is another thing; it's the creation of a force balance between opposing pitching moments from wing (wanting to pitch ND) and tail (trying to pitch NU) that allows the airplane to be statically stable at various selectable angles off the wind (angle of attack). CG is both the "pivot" of the weathervane, and is also a component of the ND pitching force required for trim.

@JohnK I used look at CG as a component of ND, but alas, to be consistent, it can always be the pivot.  Using that model, CG ahead of  CP causes ND, therefor the tail pushing down is NU!  This was a huge ASE discussion a while back, welcome to the club.

@RobertDiGiovanni The answer and comments helped me immensely, I wrongly considered the neutral point to be the CG location where the tail is producing no lift. I should keep CG slightly ahead of CP and use the trim for a level flight. At equilibrium, CG and new CP (including effects of trim) will coincide right?

@Mridul  Yes, that is correct.


text:
How critical is trailing edge geometry/sharpness on wing performance?
This question occurred to me when I was meshing an airfoil for CFD analysis having a sharp trailing edge. The upper and lower side smoothly converge having tangency at the end. After hours of struggling I realized according to our manufacturing process we will be able to achieve a minimum thinness of 0.5mm (0.02") on a wing with chord of 0.4m (1.3ft) which is near 0.25% of chord length.
My question is does difference in trailing edge thickness from razor sharp to less than 1mm cause a big difference in the performance of the airfoil? For my specific case the airfoil is operating at lower Reynolds number from 50k to 1M. I would like to know if someone has practical experience on this topic

The trailing edges on the supercritical airfoils of CRJs are squared off about 1/2 inch thick and most airliners are similar I would say.  Of course the Rn is way higher than that.

Sometimes a thick airfoil trailing edge can even reduce drag - witness US Patent 4858852A by McDonnell Douglas Corp. However, this applies to a supercritical airfoil flying at much higher Reynolds and (most likely) Mach numbers than what you have in mind.
A similar experience was already made 40 years earlier by Northrop when they could improve the flight stability of the Northrop X- Bantam by stabilizing oscillating shocks by thickening the trailing edge. The X-4 used double split flaps which opened to both sides, and by glueing a strip of balsa wood between them to keep them permanently opened at 5°, its pitch problem onset could be shifted from Mach 0.88 to Mach 0.92.
But that will not help you much if you do not intend to fly at transsonic speeds. However, a thick trailing edge is generally preferable to a razor-sharp one because it will stay straight for longer and be less of a hazard to ground crews. Since at the trailing edge you will find a several millimeters thick band of slow-moving air from the boundary layer, adding 0.5 mm from a thick trailing edge is not increasing drag significantly.
If you want to simulate the effect of a thick trailing edge: Mark Drela's XFOIL software allows to prescribe trailing edge thickness as a percentage of airfoil chord. It just so happens that XFOIL is ideally suited to study subsonic flow at the Reynolds number range you have in mind. I strongly suggest you try it out for yourself.

Quite interesting, xfoil actually shows slight improvement in cl/cd and lift coefficient on increasing TE thickness to 0.5% of chord blending from 5% chord distance from TE.


text:
Why did Concorde have a large anhedral?

Why does Concorde and many other deltas have their wings bent down?


text:
Does the flow in an afterburner have to be subsonic?
This question is prompted by the problem on page 350 of the fluid mechanics book by Landau and Lifshitz. The problem states, "A small amount of heat is supplied over a short segment of a tube in a perfect gas in steady flow in the tube. Determine the change in the gas velocity when it passes through the segment." This seems to me to be applicable to an afterburner. The solution is obtained by using conservation of mass flux and momentum flux and then increasing the energy flux by the heat added.  The change in velocity turns out to be given by,
\begin{equation}
dv=\frac{(\gamma-1)q}{\rho(c^{2}-v^{2})}
\end{equation}
where $\gamma$ is the ratio of specific heats, $q$ is heat added per unit cross-section per unit time, $\rho$ is density, $v$ is velocity and $c$ is the local speed of sound. This formula says that if the flow is supersonic $v>c$ then $dv<0$ and the gas velocity slows down. The addition of heat only increases the gas velocity if the flow is subsonic. So, my question is, "Does this result imply that the flow in an afterburner is subsonic?"

Afterburners are subsonic but not because of that equation.
Afterburners are typically subsonic for a couple of reasons. First combustion is easier at low speeds. While supersonic combustion is possible (see scramjets), most times the combustion areas of a jet are the points where some of the slowest flow through the engine occurs. Secondly, the goal of the afterburner is not to increase velocity or pressure (in fact ideal combustion is isobaric, and real combustion features a pressure loss) but to increase temperature. The reason behind this is that high temperature gas releases more energy for the same decrease in pressure than low pressure gas. This is how turbine engines are able to operate, and why combustion occurs at the highest possible pressure. After the afterburner the flow becomes supersonic traveling through the nozzle, which in an engine equipped with an afterburner is almost always variable geometry.


text:
Why do GA pilots in Europe operate N-reg (US) aircraft?
When I read articles or blogs online about GA flying in Europe I get the impression that it's not unusual for European GA pilots to have FAA licenses and fly N (US) registered aircraft.
I've never seen any numbers mentioned so I don't know how common this really is, whether they hold only FAA licenses or also European ones etc. But in general, as a European GA pilot, what is the benefit of operating an N-reg aircraft rather than a locally registered one?

It's usually cheaper to go to the US and earn an FAA license and then fly N-registered aircraft in Europe than it is to get a European license. However, an N-registered aircraft needs to be maintained to FAA standards by FAA licensed mechanics.

small detail: it's illegal in many EU countries to operate EU registered aircraft on a US license, hence the need to have US registered aircraft in Europe (and US licensed mechanics).

Thanks for the answer, do you have any information or links on the relative costs? Do European pilots have specific costs that can be avoided via FAA licensing?

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why landing gears still need bungee springs?
While studying landing gear, I came across this term 'bungee spring' to which there was no definitive explanation in text with regards to its usage.

So in the image, you can see for the jury strut/ drag brace down lock strut there are 2 bungee springs used even after the presence of downlock actuators.
As per my assumptions they must be used to give the jury strut some extra kick while extension and gravity extension and improve redundancy or there's some other reason?

I don't see any bungee springs labelled on the image, and the jury strut springs are steel springs, not bungee ones (bungee spring means a rubber chord).

Yes, I understand, in a couple of images, the springs were referred to as bungee springs so I used the same term.

Are you referring to the blue/white bungee, or the jury strut springs, or the downlock springs?

You'd have to do a detailed examination of the engineering of that landing gear design to really understand the purpose of the springs.  Certainly there are many planes that don't use bungee cords in their gear.

The key to this is the text in yellow in the bottom right-hand corner

Lower torsion link (Disassembled)

The bungee is simply being used to support part of the mechanism that is being worked on. It will be removed when the work is complete and the lower torsion link reassembled.

Are those even bungee chords and not just pieces of plain non-springy string?

It looks like bungee to me, @JanHudec, but you could be right. It doesn't affect the answer, though.

Well, either way I understood the question to be about the two springs labelled jury strut springs, which are steel springs, and the ‘bungee’ being only used due to confusion with other gear assemblies that may actually use bungee chords in similar places.

Now for that blue and white striped string, must be there to hold something in place during repairs?

The drag brace strut is an "overcenter" locking strut. It has a "knee action" to provide a kind of passive locking function because when moved into its overcenter position, compression forces a can only try to overcenter it more, which is prevented by the contact lugs above the center pivot and it can only fold going the other way.
It works the same way as your knees when standing.  You can keep your knees bent slightly so that your weight is trying to fold your legs up.  If you bring your knees back while standing as far as they will go, they "overcenter" and your weight can only try to make them overcenter more, which is prevented by the bone structure of your knees.  So when you stand up from a knee bend and your legs go into that overcenter lock mode that allows you to relax your thigh muscles, your legs are "down and locked".
While standing like that, some trickster might come along and bump your knee from behind and break the overcenter, making you start to fall before your thigh muscles can take over.
Or someone could lift you up just enough to let the compression load on your legs be released and then let you go, and might find that your knees moved forward of the overcenter axis as your weight comes back down on them, and down you go before you thigh/butt muscles take over.

The springs are there to prevent that in the case where temporary tension is applied when the gear comes under load, bringing the strut out of overcenter to on-center; when compression is re-applied, without the springs it could go either way, back into overcenter or out of overcenter, and collapsing, like your legs. The springs guarantee that the brace will always move back to the overcenter position unless forced out of it by the drag brace actuator.

The DownLock pin is used only on the ground by an engineer. It's used to prevent the undercarriage being inadvertently retracted while someone might be working on it. You can see the REMOVE BEFORE FLIGHT flag hanging down behind the Drag Brace.

I think the picture is misslabeled. The arrow with the label down lock pin is actually pointing to the pivot axel of the drag brace jury strut. The actual down lock pin is on the other side of the jury strut and does not come through the whole assembly.

@CatchAsCatchCan yes good catch.  I was fooled by the label not knowing the details of that particular gear and assumed it was a plunger actuated from the other side.  I deleted the last para.

The purpose of the (bungee) cord was correctly explained by CatchAsCatchCan... however
If the question is about the springs, their purpose is to help pull and lock the drag strut into it's down position. Once the drag brace jury strut reaches the level position in picture, the spring load is enough to keep the landing gear from collapsing under any "normal" external load in any direction.
Only way to retract the gear is to release the drag brace jury strut from it's locked position by pulling it upwards with the drag brace strut actuator. Should the actuator fail, gear lock will be achieved and maintained by the springs alone.
Please note there is a similar arrangement in the strut seen in the behind. The spring is brought outside the strut via an extension to the jury strut, and is therefore pulling upwards to lock the jury strut.

If it is the actual steel springs we are wondering about, a thought about "convertible" sofa/beds came to mind today.  Clever use of springs make these 300 lb items very easy to unfold.
Springs can be placed, with the proper leverage, to help lift or move a heavy object by opposing most of its weight, with enough weight left to keep it in place.  The counter weights of a "dumb waiter" or elevator work on the same principle.
This would serve to make the actuators much smaller and lighter, a big plus on aircraft.
Springs or "bungees" could be interchangeable, especially on smaller aircraft, but steel, especially for repeated cycles, may be favored.

I don't see how this is any more than speculation.  It doesn't attempt to answer the question.

@dalearn Think about the second spring there, and how you would lift a heavy set of gear.  No, not speculation, and not in need of a computer, either.

This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review

@Sean If you insist on piling one negative comment upon another, try reading the title of the question.  Clearly being asked is "why are there springs when there is an actuator?". Go unfold a couch and let me know what you think.

BTW if you think undamped abrupt movements are good for the actuators, remove the springs from the couch, unfold it, and let me know how your back feels.